[k] Gastroenteritis And Acid-base Disorders

THE KIDNEY

Gastroenteritis and Acid-Base Disorders

A previously healthy man develops a gastrointestinal illness with nausea and vomiting. After 12 hours of this illness, the laboratory data are shown in the table below. The illness continues, and after 60 hours the laboratory data are shown in the table below.

1. What is his acid-base disorder at 12 hours? What was its origin?

2. Has the acid-base disturbance changed after 60 hours? How do you explain the paradoxic decrease in urine pH?

The initial set of laboratory data indicates the presence of a metabolic alkalosis with appropriate respiratory compensation. Given the man's history, the most likely cause of this simple acid-base disorder is the loss of gastric acid by vomiting. The second set of laboratory data continues to show the presence of a metabolic alkalosis with respiratory compensation. In addition, fluid loss is evident (decrease in body weight by 2 kg) and, as a result, a contracted ECV (decrease in blood pressure). Given the worsening of this man's metabolic alkalosis, it is somewhat surprising that the urine pH is so acidic. The appropriate renal response should be an increase in [HCO 3-] excretion to correct the alkalosis. However, by decreasing the filtered load of [HCO3-] (decreased GFR) and stimulating proximal Na+ reabsorption, the decreased ECV prevents the excretion of [HCO3-] (HCO3- reabsorption is linked to Na+). To correct this situation, the ECV must be restored to its normal value. Infusion of isotonic NaCl would accomplish this and also allow the kidneys to excrete the excess [HCO3-], and thereby restores acid-base balance.